The present invention relates to the manufacture of integrated circuits on a substrate. More particularly, the invention relates to a method and apparatus for improving the process uniformity of plasma processing techniques used in such manufacture.
One of the primary steps in the fabrication of modem semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to generally as chemical vapor deposition (xe2x80x9cCVDxe2x80x9d). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. Plasma-enhanced CVD (xe2x80x9cPECVDxe2x80x9d) techniques, on the other hand, promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (xe2x80x9cRFxe2x80x9d) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared to conventional thermal CVD processes. These advantages are further exploited by high-density-plasma (xe2x80x9cHDPxe2x80x9d) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive.
Any of these CVD techniques may be used to deposit conductive or insulative films as necessary during the fabrication of integrated circuits. It is generally desirable that the process for depositing such a film be uniform in all respects. Recently, there has been an economically motivated trend to increase the size of circular semiconductor wafers used in such CVD applications. Currently, wafers with diameters up to 300 mm are being used, up from about 200 mm in the recent past. While the increase in wafer diameter is economically advantageous, it also tends to increase the degree of nonuniformity introduced during deposition procedures. The effects of such nonuniformity are especially noticeable when larger wafers are used because the total wafer area varies as the square of its diameter. In particular, it has been observed that the sputter nonuniformity in an HDP-CVD process is significantly greater when the process is performed on a 300-mm wafer when compared with the process performed on a 200-mm wafer. Indications suggest that if economic considerations push towards the use of even larger wafers, the effects of sputter nonuniformity will be even greater.
Accordingly, it is desirable to have a method and apparatus that will generally improve process uniformity, particularly when larger-sized wafers are to be used.
The inventors have discovered that sputter nonuniformity in plasma deposition processes is affected by magnetic fields on the order of the geomagnetic field of 0.5 gauss or less. This field can be caused by permanent magnets in the vicinity of a deposition chamber or by the earth itself. One factor in the sputter nonuniformity is believed to result from impacts from electrons in the plasma. As wafer sizes increase so that the diameters exceed the order of the mean cyclotron radius of such electrons, the effect from this factor is enhanced. Since the electron cyclotron radius is inversely proportional to the strength of the ambient magnetic field, attenuation of a magnetic field having a strength less than about 0.5 gauss within the process chamber results in an increase in the cyclotron radius of the electrons, with a concomitant decrease in the degree of sputter nonuniformity. Accordingly, in a first embodiment of the invention, a method is provided for forming a layer on a substrate during a plasma deposition process by forming a plasma in a process chamber, flowing suitable deposition precursor gases into the process chamber, and limiting sputter nonuniformity by attenuating a magnetic field having a strength less than about 0.5 gauss within the process chamber.
In specific embodiments of the invention, the attenuation of such a magnetic field is achieved with a magnetic shield positioned to enclose at least a portion of the process chamber. In some of these embodiments, the permeability of the magnetic shield is greater than 104 times the permeability of free space. In one specific embodiment, an appropriate material for the magnetic shield that achieves the desired permeability comprises greater than 75 at. % nickel and greater than 12 at. % iron; it preferably also comprises greater than 4 at. % molybdenum.
The methods of the present invention may be used with a substrate processing system. Such a substrate processing system includes a nonmagnetized substrate processing chamber and a plasma-generating system operatively coupled to the processing chamber to generate a plasma within the substrate processing chamber. A magnetic shield is configured to enclose at least a portion of the process chamber for limiting sputter nonuniformity by attenuating a magnetic field having a strength less than about 0.5 gauss within the process chamber.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.